A well recognized problem in the manufacture of a semiconductor device on a silicon wafer is the generation of impurities in a portion of the silicon wafer which becomes an active region of the semiconductor device. The impurities in the active regions of the semiconductor device may degrade the operational characteristics of the device, and may reduce total yield. A technique for reducing the impurities in the vicinity of the active regions involves collecting the impurities in portions of the silicon wafer remote with respect to the active regions. This technique is commonly referred to as gettering.
An illustrative gettering technique referred to as polysilicon back seal (PBS) is the formation of a polycrystalline silicon layer as a gettering layer on the opposed major surfaces of a silicon wafer. The polycrystalline silicon layer is typically formed by growing polycrystalline silicon on the silicon wafer utilizing a low pressure chemical vapor deposition (LPCVD) method. The grain size difference between the silicon wafer and the polycrystalline layer causes stress in the structure which acts as a "getter" that draws nearby impurities to the sites of stress. The silicon wafer can then have one side polished such that the resulting wafer has one major surface that has a polycrystalline silicon layer and the other major surface is substantially free of impurities.
In order to grow the polycrystalline silicon layer on a silicon wafer, a LPCVD furnace may be utilized. LPCVD furnaces have been widely used in the microelectronics industry for years for thin film deposition and other thermal processing steps required in the manufacture of integrated circuits. The evolution of these furnaces has largely been driven by the need for improved process uniformity and high wafer throughput. The need for high process uniformity refers to both wafer-to-wafer and within-wafer uniformity. An example of a commercially available LPCVD furnace is Model 7000+ by Thermco Systems, Orange, Calif., USA. In general, advantages of LPCVD techniques may include uniform step coverage, precise control of composition and structure, low temperature processing, fast deposition rates, high throughput, and low processing cost.
A vertical LPCVD furnace typically includes a vertically positioned bell-shaped quartz tube that is approximately 40 inches in length. Within the quartz tube, a quartz pedestal located at the bottom of the quartz tube supports a wafer boat holding any number of horizontally positioned silicon wafers. In order to grow polycrystalline layers on the silicon wafers within the quartz tube, the furnace is heated and one or more deposition gases, such as silane (SiH.sub.4), are introduced into the quartz tube. The deposition gases are typically introduced through both an upper gas injection tube and a lower gas injection tube. The upper gas injection tube enters the quartz tube at its belled end, and typically extends a couple of inches into the interior chamber of the quartz tube, if it extends into the interior chamber at all. The lower gas injection tube also enters the quartz tube at its belled end, and typically extends approximately 30 or more inches in the quartz tube toward the bottom of the quartz tube. Thus, an inlet at the distal end of the lower gas injection tube is proximate the bottom of the wafer boat. An illustrative example of a conventional LPCVD furnace is provided in U.S. Pat. No. 5,076,206 to Bailey et al.
Accordingly, the deposition gases may be introduced simultaneously into the quartz tube near the top and bottom of the wafer boat by means of the upper and lower gas injection tubes respectively. This facilitates substantially uniform growth of the polycrystalline layer on silicon wafers. In operation, silane flows into the heated quartz tube via the upper gas injection tube and the lower gas injection tube. The silane gas is heated as it enters the quartz tube and deposits on the surfaces of the silicon wafers to form a polycrystalline layer on the exposed surfaces of the respective silicon wafers.
Over time, however, silane deposits on the inside surfaces of the lower gas injection tube, which will restrict the flow of silane into the quartz tube. The lower gas injection tube experiences a greater amount of deposition than the upper gas injection tube because of its length. As a consequence, the precision with which predetermined quantities of silane are introduced into the quartz tube by the lower gas injection tube may be reduced.
The build up of silane on the lower gas injection tube also contributes to the premature breaking or cracking of the injection tube due to, among other reasons, differences in the coefficients of thermal expansion between the quartz tube and the material which forms within the injection tube. In addition, the cracking and breaking of the lower injection tube may require expensive maintenance and repair, and may ultimately reduce yield.
Further, the fabrication of the lower gas injection tube requires a relatively high degree of skill and precision, and therefore, is a costly component of the quartz tube. In particular, the lower gas injection tube is connected to the quartz tube at the bell-shaped portion of the quartz tube, which makes it difficult to attach the lower gas injection tube to the quartz tube. The length and positioning of the injection tube within the quartz tube requires precise alignment in order to have the tube run parallel with the wall of the quartz tube. Thus, the lower gas injection tube is a critical design feature of any quartz tube that is being utilized in polysilicon backseal operations.
Therefore, an unresolved need exists in the industry for a LPCVD furnace capable of injecting a deposition gas at the upper and lower portions of a quartz tube while advantageously reducing maintenance and fabrication cost while increasing yield.